1. Field of the Invention
The present invention relates to a switch control device capable of preventing a malfunction caused by a leading edge current (LEC), a switch control method, and a converter using the same.
2. Description of the Related Art
A converter is a power device that converts AC and DC signals. A converter may include an AC/DC converter that converts AC into DC, a DC/DC converter that converts DC into DC, and an inverter that converts DC into AC signals. A typical use of a converter is for a switching mode power supply (SMPS).
In general, a converter can adjust a drain-source current Ids flowing through a main switch by controlling a turn-off time of the main switch. The converter can use output voltage information corresponding to the load at an output terminal, thereby generating a more uniform, regulated output voltage. In order to prevent the main switch from being damaged by an overload or a short circuit at the output terminal, a maximum current limit ILIM can be set. When the Ids current reaches the maximum current limit ILIM, the main switch can be turned off. In this case, however, the main switch is turned off after an inevitable current limit delay time TCLD, caused by various circuit elements of the converter from the time at which the current Ids reaches the maximum current limit ILIM. The TCLD can be caused, for example, by an internal propagation delay time of a controller that controls an ON/OFF operation of the main switch and a turn-off delay time of the main switch.
Meanwhile, when the main switch is turned on, a leading edge current (LEC) may be generated, a phenomenon where the current Ids sharply increases and then drops due to a parasitic capacitance of the transformer and the switch. Therefore, the conventional converters include a leading edge blanking (LEB) circuit in order to prevent a malfunction caused by the LEC. The LEB circuit can perform an LEB operation such that the current Ids is not sensed during a time interval in which the LEC is generated, namely, during the leading edge blanking time, or LEB time. Conventional converters may also include an abnormal overcurrent protection (AOCP) circuit that can turn off the main switch when it senses an overcurrent possibly generated during the LEB time.
However, in using the LEB circuit, the current Ids cannot be sensed until the LEB operation ends after the main switch is turned on, and so a minimum turn-on state maintaining time (referred to as ‘Tmin.on’ hereinafter) of the main switch is approximately equivalent to the sum of the TCLD and the LEB time. In other words, the use of the LEB circuit lengthens the Tmin.on by the LEB time, compared with the case where the LEB circuit is not used. A problem associated with the lengthening of the Tmin.on will now be described with reference to FIGS. 1 and 2.
FIG. 1 illustrates a waveform of the current Ids when the main switch of a conventional converter is turned on or off in a normal state. Here, the normal state means that the converter output terminal is not overloaded or shorted.
The LEC is generated at a time T1 when the main switch of the conventional converter is turned on. Thereafter, during the LEB operation of the LEB circuit, the controller of the converter does not sense the current Ids during the LEB time from T1 to T2. In the normal state, the controller of the converter can compare the current Ids with a current corresponding to a feedback information, and when it senses that the Ids reaches the signal level of the feedback information, the controller turns off the main switch.
When the controller senses that the Ids has exceeded the current corresponding to the feedback information at the time T2 at which the LEB time expires, the controller turns off the main switch. In response, the main switch is turned off at a time T3, delayed by TCLD from the time T2. The Ids has a peak value IPEAK at the time T3 at which the main switch is turned off. Notably, the current Ids after the LEB time increases with a steeper slope as the voltage supplied to the converter increases, so the higher the input voltage becomes, the higher a current IDIFF becomes. Here, the IDIFF is a difference between IPEAK and the current corresponding to the feedback information at the time T3. This can cause serious problems and prevent conventional converters from properly controlling the current Ids, as shown in FIG. 2.
FIG. 2 illustrates a change in the current Ids according to the ON/OFF operation of the main switch of conventional converters when the output terminal of the converter is overloaded or short-circuited. When the output terminal of the converter is overloaded or short-circuited, the current Ids after the LEC appears much larger than the Ids in the normal state as shown in FIG. 1. However, in conventional converters, the main switch cannot be turned off during the Tmin.on, causing the peak value IPEAK of the Ids to exceed the maximum current limit ILIM, which can result in possible damage of the main switch.
Recently, intense research has been performed to reduce the size and cost of converters and the converter controller. However, it has been very difficult to obtain a compact and low-cost converter and converter controller as long as the conventional converters included the LEB circuit and the AOCP circuit for preventing the malfunction caused by the LEC.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.